(a) Field of the Invention
The present invention relates to a large aperture ratio zoom lens system and, more particularly, to a large aperture ratio zoom lens system in which at least one of lens surfaces in formed as an aspherical surface and which is arranged to comprise a small number of lenses.
(b) Description of the Prior Art
Most of known zoom lens systems for electronic photographing cameras adopting image pick-up tubes or solid-state image pick-up devices generally have aperture ratios of F/2.0 and over and zoom ratios of 3 and over. However, the above-mentioned kind of zoom lens systems comprise large numbers of lens elements. For example, the zoom lens systems according to the U.S. patent application No. 706,863 (Japanese patent application Nos. 38649/84 and 41974/84) invented by the same inventor as the present invention comprises thirteen or fourteen lens elements. The above-mentioned zoom lens system is designed so as to have high performance by expecting that the performances of image pick-up devices will become higher. Moreover, when the technology for the manufacture of aspherical lenses progresses in future so that aspherical lenses can be obtained at low costs, it will be possible to obtain a zoom lens system having a performance equivalent to or higher than that of the zoom lens system according to the above-mentioned invention (U.S. patent application No. 706,863) but comprising a smaller number of lens elements. In other words, it will be possibe to obtain a zoom lens system with a performance higher than that of the zoom lens system accoridng to said invention at a lower cost.